Studies were conducted to determine control mechanisms regulating the phagocytic activity of retinal pigmented epithelium. Cells from normal and retinal dysplastic animals were grown in culture and tested using two different approaches. Radioactively labeled photoreceptor outer segments from normal dogs were incubated with cultures of normal Irish setter, retinal dystrophic Irish setter, normal monkey, chick, and human ocular cells in culture. The outer segments were ingested by all the cells to which they were exposed, not just the pigmented epithelium. The cells from dystrophic animals ingested significantly fewer labeled outer segments. In another series experiments, the membrane receptors that have been documented to be on the cells of the reticuloendothelial system were sought on freshly prepared and cultured cells from normal and dystrophic setters. As judged by rosette formation with immunologically labeled sheep erythrocytes, the Fc receptor and possibly the C3 receptor systems are present.